This invention relates to gerotor fuel pumps and, more particularly, to pumps with pressure balancing of the rotors for reduced wear.
Generally in a gerotor pump, a pressure imbalance between a high pressure discharge side of the inner and outer rotors and a low pressure inlet side of the rotors is present, generating forces that tend to tip or bias the rotors against one of the adjacent side plates. This may be acceptable where the pump is used for pressurizing lubricating oil in an engine because the rotors develop hydrodynamic lubricating films which may be adequate to prevent rubbing of the rotors on the side plates and thereby avoid excessive wear.
However, when a gerotor pump is used to pressurize gasoline, the extremely low viscosity of this fluid makes it difficult to establish hydrodynamic lubrication at high outlet pressures. Without this form of lubrication, higher cost material must be used or other more complex lubrication systems would be required in order to prevent excessive wear. Also, high operating pressure increases the internal leakage of the pump and reduces the volumetric efficiency, resulting in an impractical pump for automotive applications as a fuel pump. Operating pressures for gerotor gasoline pumps have accordingly been limited to relatively low pressures, typically below 1.0 MPa.
The present invention provides a gerotor pump for pressurizing gasoline fuel and capable of developing pressures up to 2.0 MPa with good mechanical and volumetric efficiency and satisfying the durability requirements for an automotive fuel pump. The pump has been designed with optimized clearances and by including features that promote the formation of lubricating films of pressurized fuel.
A feature of the improved pump is the use of a shadow port in the side plate opposite the outlet port and arranged to promote balancing of high fuel pressures on the opposite sides of the rotors.
A further preferred feature is that the inner and outer rotors have predetermined side clearances. The clearances of the outer rotor are greater than those of the inner rotor in order to promote fuel pressure balance on the sides of the outer rotor.
An additional preferred feature is inclusion of a central recess in the side portion opposite to the side which supports a drive shaft and open to a side of the inner rotor surrounding the drive shaft. The recess communicates through a restricted passage with outlet pressure from the adjacent shadow port for assisting force balance on opposite sides of the inner rotor.
Still another preferred feature is that the drive shaft and the inner rotor are both supported by a single bushing mounted in a side portion of the housing. A first bearing sleeve supports the drive shaft in the bushing and a second bearing sleeve supports the inner rotor on an outer diameter of the bushing.
An optional feature is that the bushing extends into a recess in the inner rotor which communicates with the outlet port through restricted clearances between the inner rotor and the side plate which supports the drive shaft and between the bushing and a bearing sleeve in the recess.
An optional additional feature is that a hard coating such as chromium may be applied to the faces of the side plates to minimize wear when the pump is starting, stopping or running at a speed too low to develop a satisfactory hydrodynamic lubricating film.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.